The place I worked at would regularly get molding compound for encapsulating electronic modules. The material was shipped in Styrofoam coolers filled with dry ice pellets. I would take home a whole cooler filled with the pellets, dig out every mole hill I could find and stuff a good batch of dry ice in there. Plug the hole with dirt and move on to the next. Dry ice turns into a very large volume of gas (22.4 liters per mole of dry ice, about 44 grams) and it's heavier than air so will persist awhile in a tunnel. That seemed to greatly reduce the mole activity around the place. And the CO2 doesn't explode or poison you. I'm not claiming it won't asphyxiate you, but that would be hard to accomplish in this kind of situation.
When I was in high school the local hardware store sold a product to kill moles and gophers. Called "gopher gas", it was granulated calcium carbide (oh yeah!). Put a few tablespoons in a hole, add water and cover. The carbide would release acetylene gas to asphyxiate the critters. Naturally, I had the bright idea of taking it one step further and igniting the stuff. I only did that once. The explosion blew a rather large plug of dirt out of the hole, and that also is when I learned that gophers dig their tunnels with multiple escape holes. It turns out one was in a recently-harvested wheat field behind our house. I looked up and saw smoke curling up in the stubble, but I was able to stomp it out before the whole field went up. That incident is one of the star entries in the list of things I never told my parents about. There are others, but my lips are sealed